Merge -onoff proxy code into minor mode function

Merge -onoff proxy code into minor mode function

This simplifies bookkeeping and keeping the state of locally cached
servers, their managed buffers, and the buffer-local mode consistent.

The "on" case of the -onoff code now expects that
`eglot--cached-current-server' has been set already, the "off" case uses
the same value.

* eglot.el (eglot--managed-mode-onoff): Remove.
(eglot--managed-mode): Adopt code.
(eglot--managed-mode-off): New minimal wrapper.